In the usual color cathode ray tube, three electron beams (designated R, G, B beams for the colors red, green, and blue) shot by an electron gun are controlled to move across the tube screen in the required direction by magnetic fields generated in a deflection yoke. Then, the beams pass through apertures in a shadow mask, and collide with phosphor dots formed on the screen, thereby forming pictures.
However, sometimes the center axis of the coil assembly of the deflection yoke does not correspond to the center axis of the electron beam. Moreover, sometimes the strength and characteristics of the electron beams are not uniform due to imperfect distribution by the electron gun. Due to these and other factors, the three electron beams may fail to converge properly at the central portion of the picture, a phenomenon called misconvergence which causes color misalignment, thereby deteriorating the picture quality.
In an attempt to correct such misconvergence, Japanese Utility Model Publication No. Sho 58-99755 proposes a method in which the winding method for the coil of the deflection yoke is modified.
Another convergence correction method is known, using a color purity enhancing magnet or a ring magnet having multiple poles attached on the circumference of the holder portion of the CRT deflection yoke, so that the position of the electron beams can be adjusted by the magnetic fields of the ring magnet, thereby obtaining high quality convergence.
Conventionally, ring magnets having 4 or 6 poles are attached around the color purity enhancing magnet on the holder portion of the deflection yoke, and this assembly is attached on the neck portion of the cathode ray tube in order to obtain high quality convergence. However, in this type of magnet convergence correction device, the flux densities of the magnets are fixed at a predetermined level, and therefore, obtaining a precise convergence correction is difficult because the beams may drift over time. It is also difficult to maintain the beams in a corrected position because the ring magnet can rotate and the correction is not precise.